Configuration of external validation endpoint

[leoraba]

Summary of request

Validation endpoint requires a basic configuration in order to perform checks of ID fields.

Details

• Add required configuration properties to AppConfig and BaseDependencies . For instance validator should contain an array of category, entity and field to enable search.
• To make sure these configuration is valid we need to validate category, entity and field against the dictionary on that category
• Combination of category and entity should be unique
• Every request into the validator endpoint should check if category, entity and field exists in configuration, If not, return 404 Not found response.

Related to issues:

#109